1. Field of the Invention
The present invention relates to a compact printing apparatus for use in a measuring apparatus, medical equipment, a POS (point-of-sale) terminal, a telephone and the like.
2. Description of the Related Art
In various industrial fields, higher quality has been demanded for a printing apparatus as well as compactness and inexpensiveness.
In the field of thermal printers, the number of components and the number of production processes has been reduced to comply with the demand for lower prices. Moreover, printers having low noise and multiple functions are demanded as the uses thereof are varied. Regarding quality, drawings and graphs are now printed in addition to letters. More precise paper feeding is required for accurate drawings and graphs.
Referring briefly to FIG. 10, there is shown an exploded isometric view of a conventional printing apparatus. The printing apparatus 100 of FIG. 10 includes a frame 1 formed of metal having a bottom wall. Side walls 2 and 3 are fixed to the frame 1. A paper guiding plate 8 for guiding thermal paper used as recording paper 16 to a printing position, a platen holder 15 for supporting a platen 17 so that the platen 17 can be pivoted, and a sliding shaft 7 for guiding a carriage 6 are supported by the side walls 2 and 3. A rack 4 having a cam slope 4b is attached to the frame 1. A printing head 5 is supported by the carriage 6. The carriage 6 also has a stepping motor 6a provided on a top surface thereof, and a gear 6b directly connected to the stepping motor 6a and a head-up cam 6c provided on a bottom surface thereof. The gear 6b is engaged with the rack 4 by springs 4a, and the head-up cam 6c is to be rotated against and slid on the cam slope 4b. The printing head 5 is pressed on the recording paper 16 by a coil spring 5a provided between the carriage 6 and the printing head 5. The printing head 5 is supplied with electric energy by a flexible printed circuit board 5b (hereinafter, referred to as the "FPC") for printing.
Leaf springs 9 are welded to the paper guiding plate 8 so as to press the recording paper 16 on paper feeding rollers 10a formed of rubber. The paper feeding rollers 10a are provided at specified positions of a paper feeding roller shaft 10, integrally therewith. A wheel 11 having radial teeth 11a is fixed to the paper feeding roller shaft 10. A ratchet gear 12 is rotatably provided inside the wheel 11. The ratchet gear 12 has teeth which are engaged with the radial teeth 11a by a compression spring 13. The ratchet gear 12 is also engaged with a rack gear 14a formed at an end of a cam plate 14. The ratchet gear 12 and the rack gear 14a act as a one-way clutch mechanism rotating in one direction. The cam plate 14 has a groove 14b engaged with a pin (not shown) formed on the carriage 6. As is illustrated, one end (curved portion) 14c of the groove 14b is curved or bent.
The operation of the printing apparatus having the above-mentioned construction will be described with reference to FIGS. 11a and 11b, which shows the relationship between the movement of the carriage 6 and the operation of the printing head 5.
The recording paper 16 is inserted from the rear side of the paper guiding plate 8 and fed between the pinching leaf springs 9 and the paper feeding rollers 10a onto a forward surface of the platen 17.
Then, the stepping motor 6a is driven to rotate. Due to the engagement of the gear 6b connected to the stepping motor 6a and the rack 4, the rotation of the stepping motor 6a is transmitted to the carriage 6, thereby moving the carriage 6 along the sliding shaft 7 rightward as shown in FIG. 11B. While the carriage 6 moves rightward, the printing head 5 is supplied with electric energy in phase with the movement of the carriage 6, thereby performing printing on the thermal paper used as the recording paper 16. When printing of one line is completed, the carriage 6 moves leftward as shown in FIG. 11A and the cam plate 14 is pivoted due to the engagement of the pin formed on the carriage 6 and the curved portion 14c of the groove 14b. Then, the ratchet gear 12 in engagement with the rack gear 14a is rotated by a certain distance, and the rotation thereof is transmitted to the wheel 11. Then, the paper feeding rollers 10a integral with the wheel 11 are rotated by a certain distance to advance the recording paper 16. The headup cam 6c is rotated against and slid along the cam slope 4b to separate the printing head 5 from the platen 17. When reaching an end of the cam slope 4b, the head-up cam 6c is rotated oppositely by a returning spring (not shown) in a separation stabilization area; this operation of the head-up cam 6c presses the printing head 5 on the recording paper 16.
The above-mentioned printing apparatus has a large number of components, and thus production cost is high. Further, there are some problems concerning precision of paper feeding.
One of the problems occurs when the recording paper 16 is cut. Since the ratchet gear 12 and the rack gear 14a act as a one-way clutch mechanism, the recording paper 16 is pulled freely, that is an extra amount of the recording paper 16 is pulled out. Further, due to the one-way clutch mechanism, the ratchet gear 12 and the rack gear 14a are displaced with respect to each other, and thus the margin can vary for the top line of the next page.
Another problem concerning precision of paper feeding lines occurs because of the pivotable platen 17. While the recording paper 16 is fed, the slanting angle of the platen 17 changes by the friction between the platen 17 and the recording paper 16, thereby loosening the recording paper 16. As a result, the margin can vary for the top line of the next page. Further, when the head-up cam 6c is rotated oppositely for pressing the printing head 5 on the recording paper 16, the force of the coil spring 5a suddenly acts, thereby making a noise.